Holographic image projection with holographic correction

ABSTRACT

There is provided a method of projection using an optical element (502,602) having spatially variant optical power. The method comprises combining Fourier domain data representative of a 2D image with Fourier domain data having a first lensing effect (604a) to produce first holographic data. Light is spatially modulated (504,603a) with the first holographic data to form a first spatially modulated light beam. The first spatially modulated light beam is redirected using the optical element (502,602) by illuminating a first region (607) of the optical element (602) with the first spatially modulated beam. The first lensing effect (604a) compensates for the optical power of the optical element in the first region (607). Advantageous embodiments relate to a head-up display for a vehicle using the vehicle windscreen (502,602) as an optical element to redirect light to the viewer (505,609).

This application is a continuation of U.S. patent application Ser. No.14/654,275 filed on Jun. 19, 2015, which is hereby incorporated hereinby reference in its entirety. U.S. patent application Ser. No.14/654,275 is a U.S. national phase application of International PatentApplication no. PCT/GB2013/053403 filed on Dec. 20, 2013, which claimsthe benefit of United Kingdom Patent Application no. GB 1223416.7 filedon Dec. 21, 2012. The benefit of priority of the above-referencedapplications is hereby claimed.

FIELD OF THE INVENTION

The present disclosure relates to the field of image projection.Embodiments disclosed herein generally relate to holographic imageprojection and a method for the same. More specifically, embodimentsdisclosed herein generally relate to a head-up display and a method ofprojecting holographic images using a windscreen.

BACKGROUND

Light scattered from an object contains both amplitude and phaseinformation. This amplitude and phase information can be captured on,for example, a photosensitive plate by well-known interferencetechniques to form a holographic recording, or “hologram”, comprisinginterference fringes. The “hologram” may be reconstructed byilluminating it with suitable light to form a holographicreconstruction, or replay image, representative of the original object.

It has been found that a holographic reconstruction of acceptablequality can be formed from a “hologram” containing only phaseinformation related to the original object. Such holographic recordingsmay be referred to as phase-only holograms. Computer-generatedholography may numerically simulate the interference process, usingFourier techniques for example, to produce a computer-generatedphase-only hologram. A computer-generated phase-only hologram may beused to produce a holographic reconstruction representative of anobject.

The term “hologram” therefore relates to the recording which containsinformation about the object and which can be used to form areconstruction representative of the object. The hologram may containinformation about the object in the frequency, or Fourier, domain.

It has been proposed to use holographic techniques in a two-dimensionalimage projection system. An advantage of projecting images usingphase-only holograms is the ability to control many image attributes viathe computation method—e.g. the aspect ratio, resolution, contrast anddynamic range of the projected image. A further advantage of phase-onlyholograms is that no optical energy is lost by way of amplitudemodulation.

A computer-generated phase-only hologram may be “pixellated”. That is,the phase-only hologram may be represented on an array of discrete phaseelements. Each discrete element may be referred to as a “pixel”. Eachpixel may act as a light modulating element such as a phase modulatingelement. A computer-generated phase-only hologram may therefore berepresented on an array of phase modulating elements such as a liquidcrystal spatial light modulator (SLM). The SLM may be reflective meaningthat modulated light is output from the SLM in reflection.

Each phase modulating element, or pixel, may vary in state to provide acontrollable phase delay to light incident on that phase modulatingelement. An array of phase modulating elements, such as a Liquid CrystalOn Silicon (LCOS) SLM, may therefore represent (or “display”) acomputationally-determined phase-delay distribution. If the lightincident on the array of phase modulating elements is coherent, thelight will be modulated with the holographic information, or hologram.The holographic information may be in the frequency, or Fourier, domain.

Alternatively, the phase-delay distribution may be recorded on akinoform. The word “kinoform” may be used generically to refer to aphase-only holographic recording, or hologram.

The phase delay may be quantised. That is, each pixel may be set at oneof a discrete number of phase levels.

The phase-delay distribution may be applied to an incident light wave(by illuminating the LCOS SLM, for example) and reconstructed. Theposition of the reconstruction in space may be controlled by using anoptical Fourier transform lens, to form the holographic reconstruction,or “image”, in the spatial domain. Alternatively, no Fourier transformlens may be needed if the reconstruction takes place in the far-field.

A computer-generated hologram may be calculated in a number of ways,including using algorithms such as Gerchberg-Saxton. TheGerchberg-Saxton algorithm may be used to derive phase information inthe Fourier domain from amplitude information in the spatial domain(such as a 2D image). That is, phase information related to the objectmay be “retrieved” from intensity, or amplitude, only information in thespatial domain. Accordingly, a phase-only holographic representation ofan object in the Fourier domain may be calculated.

The holographic reconstruction may be formed by illuminating the Fourierdomain hologram and performing an optical Fourier transform, using aFourier transform lens, for example, to form an image (holographicreconstruction) at a reply field such as on a screen.

FIG. 1 shows an example of using a reflective SLM, such as a LCOS-SLM,to produce a holographic reconstruction at a replay field location, inaccordance with the present disclosure.

A light source (110), for example a laser or laser diode, is disposed toilluminate the SLM (140) via a collimating lens (111). The collimatinglens causes a generally planar wavefront of light to become incident onthe SLM. The direction of the wavefront is slightly off-normal (e.g. twoor three degrees away from being truly orthogonal to the plane of thetransparent layer). The arrangement is such that light from the lightsource is reflected off a mirrored rear surface of the SLM and interactswith a phase-modulating layer to form an exiting wavefront (112). Theexiting wavefront (112) is applied to optics including a Fouriertransform lens (120), having its focus at a screen (125).

The Fourier transform lens (120) receives a beam of phase-modulatedlight exiting from the SLM and performs a frequency-space transformationto produce a holographic reconstruction at the screen (125) in thespatial domain.

In this process, the light—in the case of an image projection system,the visible light—from the light source is distributed across the SLM(140), and across the phase modulating layer (i.e. the array of phasemodulating elements). Light exiting the phase-modulating layer may bedistributed across the replay field. Each pixel of the hologramcontributes to the replay image as a whole. That is, there is not aone-to-one correlation between specific points on the replay image andspecific phase-modulating elements.

The Gerchberg Saxton algorithm considers the phase retrieval problemwhen intensity cross-sections of a light beam, I_(A)(x,y) andI_(B)(x,y), in the planes A and B respectively, are known and I_(A)(x,y)and I_(B)(x,y) are related by a single Fourier transform. With the givenintensity cross-sections, an approximation to the phase distribution inthe planes A and B, Φ_(A)(x,y) and Φ_(B)(x,y) respectively, is found.The Gerchberg-Saxton algorithm finds solutions to this problem byfollowing an iterative process.

The Gerchberg-Saxton algorithm iteratively applies spatial and spectralconstraints while repeatedly transferring a data set (amplitude andphase), representative of I_(A)(x,y) and I_(B)(x,y), between the spatialdomain and the Fourier (spectral) domain. The spatial and spectralconstraints are I_(A)(x,y) and I_(B)(x,y) respectively. The constraintsin either the spatial or spectral domain are imposed upon the amplitudeof the data set. The corresponding phase information is retrievedthrough a series of iterations.

A holographic projector may be provided using such technology. Suchprojectors have found application in head-up displays for vehicles.

The use of head-up displays in automobiles is becoming increasingpopular. Head-up displays are broken down in to two main categories,those which use a combiner (a free standing glass screen whose purposeis to reflect a virtual image in to the driver's line of sight) andthose which utilise the vehicle's windscreen to achieve the samepurpose.

FIG. 2 shows an example head-up display comprising a light source 206, aspatial light modulator 204 arranged to spatially modulate light fromthe light source with holographic data representative of an image forprojection, a Fourier transform optic 205, a diffuser 203, a freeformmirror 201, a windscreen 202 and a viewing position 207. FIG. 2 shows aso-called “indirect view” system in which a real image of theholographic reconstruction is formed at a replay field on the diffuser203. A holographic reconstruction is therefore projected on the diffuser203 and may be viewed from viewing position 207 by focusing on thediffuser 203. The projected image is viewed via a first reflection offfreeform mirror 201 and a second reflection off windscreen 202. Thediffuser acts to increase the numerical aperture of the holographicsystem, fully illuminating the freeform mirrors thereby allowing thevirtual image to be viewed by a driver, for example.

However, a problem with using a windscreen 202 as a so-called “combiner”is that the curvature of the windscreen applies lensing power to thevirtual image being displayed. This problem is further complicated bythe different windscreen curvatures 202 that exist from left to right &top to bottom. Normally this complex lensing function is correctedthrough the use of a carefully designed freeform mirror 201. However,these mirrors are extremely complex to design with minimal aberrationsand are extremely costly to manufacture with the required precision.

The present disclosure aims to address these problems and provide animproved projector.

SUMMARY OF THE INVENTION

Aspects of an invention are defined in the appended independent claims.

There is provided an improved method of projection of a target image. Inparticular, there is provided a method of projection using an opticalelement having spatially varying optical power such as a vehiclewindscreen. The optical power of the optical element is compensated bycombining image-content data with data having a lensing effect.Advantageously, a system is provided which can adjustably compensate forthe irregular optical component.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described to the accompanying drawings in which:

FIG. 1 is a schematic showing a reflective SLM, such as a LCOS, arrangedto produce a holographic reconstruction at a replay field location;

FIG. 2 shows a so-called “indirect view” holographic projector for ahead-up display of a vehicle;

FIG. 3 shows an example algorithm for computer-generating a phase-onlyhologram;

FIG. 4 shows an example random phase seed for the example algorithm ofFIG. 3;

FIG. 5 shows one channel of a “direct view” head-up display for avehicle;

FIG. 6 show a “direct view” for a head-up display of a vehicle inaccordance with embodiments;

FIG. 7 is an algorithm for calculating a Fresnel hologram in accordancewith embodiments; and

FIG. 8 is a schematic of a LCOS SLM.

In the drawings, like reference numerals referred to like parts.

DETAILED DESCRIPTION OF THE DRAWINGS

Holographically-generated 2D images are known to possess significantadvantages over their conventionally-projected counterparts, especiallyin terms of definition and efficiency.

Modified algorithms based on Gerchberg-Saxton have been developed—see,for example, co-pending published PCT application WO 2007/131650incorporated herein by reference.

FIG. 3 shows a modified algorithm which retrieves the phase informationw[u,v] of the Fourier transform of the data set which gives rise to aknown amplitude information T[x,y] 362. Amplitude information T[x,y] 362is representative of a target image (e.g. a photograph). The phaseinformation ψ[u,v] is used to produce a holographic representative ofthe target image at an image plane.

Since the magnitude and phase are intrinsically combined in the Fouriertransform, the transformed magnitude (as well as phase) contains usefulinformation about the accuracy of the calculated data set. Thus, thealgorithm may provide feedback on both the amplitude and the phaseinformation.

The algorithm shown in FIG. 3 can be considered as having a complex waveinput (having amplitude information 301 and phase information 303) and acomplex wave output (also having amplitude information 311 and phaseinformation 313). For the purpose of this description, the amplitude andphase information are considered separately although they areintrinsically combined to form a data set. It should be remembered thatboth the amplitude and phase information are themselves functions of thespatial coordinates (x,y) for the farfield image and (u,v) for thehologram, both can be considered amplitude and phase distributions.

Referring to FIG. 3, processing block 350 produces a Fourier transformfrom a first data set having magnitude information 301 and phaseinformation 303. The result is a second data set, having magnitudeinformation and phase information ψ_(n)[u,v] 305. The amplitudeinformation from processing block 350 is set to a distributionrepresentative of the light source but the phase information ψ_(n)[u,v]305 is retained. Phase information 305 is quantised by processing block354 and output as phase information ψ[u,v] 309. Phase information 309 ispassed to processing block 356 and combined with the new magnitude byprocessing block 352. The third data set 307, 309 is applied toprocessing block 356 which performs an inverse Fourier transform. Thisproduces a fourth data set R_(n)[x,y] in the spatial domain havingamplitude information 311 and phase information 313.

Starting with the fourth data set, its phase information 313 forms thephase information of a fifth data set, applied as the first data set ofthe next iteration 303′. Its amplitude information R_(n)[x,y] 311 ismodified by subtraction from amplitude information T[x,y] 362 from thetarget image to produce an amplitude information 315 set. Scaledamplitude information 315 (scaled by α) is subtracted from targetamplitude information T[x,y] 362 to produce input amplitude informationη[x,y] 301 of the fifth data set for application as first data set tothe next iteration. This is expressed mathematically in the followingequations:R _(n+1)[x,y]=F′{exp(iψ _(n)[u,v])}ψ_(n)[u,v]=∠F{η·exp(i∠R _(n)[x,y])}η=T[x,y]−α(|R _(n)[x,y]|−T[x,y])Where:

-   F′ is the inverse Fourier transform.-   F if the forward Fourier transform.-   R is the replay field.-   T is the target image.-   ∠ is the angular information.-   ψ is the quantized version of the angular information.-   ε is the new target magnitude, ε≥0-   α is a gain element ˜1

The gain element α may be predetermined based on the size and rate ofthe incoming target image data.

In the absence of phase information from the preceding iteration, thefirst iteration of the algorithm uses a random phase generator to supplyrandom phase information as a starting point. FIG. 4 shows an examplerandom phase seed.

In a modification, the resultant amplitude information from processingblock 350 is not discarded. The target amplitude information 362 issubtracted from amplitude information to produce a new amplitudeinformation. A multiple of amplitude information is subtracted fromamplitude information 362 to produce the input amplitude information forprocessing block 356. Further alternatively, the phase is not fed backin full and only a portion proportion to its change over the last twoiterations is fed back.

Accordingly, Fourier domain data representative of an image of interestmay be formed. Embodiments relate to phase-holograms by way of exampleonly and it may be appreciated that the present disclosure is equallyapplicable to amplitude holograms.

In summary, the inventors have recognised that problems caused by usinga combiner having a spatially-varying optical power, such as a vehiclewindscreen, may be addressed by using a so-called “direct view” system,instead of an “indirect view” system, and combining the Fourier domaindata representative of the image with Fourier domain data having alensing effect which compensates for the optical power added by thecombiner. The data may be combined by simple addition. In this respect,the hologram comprises first data representative of the actual image forprojection and second data comprising a lensing function. In particular,this approach allows for real-time adjustment of the compensation if,for example, the projection system is realigned during use and adifferent region of the combiner is used. Such realignment may berequired if a viewer moves, for example.

FIG. 5 shows a so-called “direct view” system for a head-up displaycomprising a light source 501, a SLM 504, a freeform mirror 503, awindscreen 502 and a viewing position 505. Notably, the lens in viewer'seye performs the necessary Fourier Transform. A direct view system doesnot therefore comprise a Fourier lens. If the rays from the SLM arecollimated then the eye will need to focus at infinity for a sharp imageto form on the retina. However, if a Fourier domain data comprising alensing effect is added to the Fourier domain data representative of theimage, the light rays will cease to be collimated and the eye will needto focus at the focal length defined by the lensing effect for a sharpreplay field to be formed on the retina.

In an embodiment, Fourier domain data having a lensing effect iscombined—for example, added—to the Fourier domain data represented ofthe image for projection to compensate, or even negate, the impact ofthe optical power of the windscreen. The skilled person knows how tocalculate Fourier domain data having a required lensing effect and howto add such data to other Fourier domain data.

There is therefore provided a method of projection using an opticalelement having spatially variant optical power, the method comprising:combining Fourier domain data representative of a 2D image with Fourierdomain data having a first lensing effect to produce first holographicdata; spatially modulating light with the first holographic data to forma first spatially modulated light beam; redirecting the first spatiallymodulated light beam using the optical element by illuminating a firstregion of the optical element with the first spatially modulated beam;wherein the first lensing effect compensates for the optical power ofthe optical element in the first region.

Given that the SLM may have a low numerical aperture, the holographicreconstruction will only be visible to one eye. Therefore, in a furtheradvantageous embodiment, two SLMs are used to provide two holographicprojections. See FIG. 6. As each eye 609 will view a differentprojection, each projection reflects off of a different area, or region,of the windscreen 606 and 607. Each area is likely to have a differentoptical power and this can therefore be corrected, or compensated for,individually. Notably, the inventors have recognised that the differentprojections are affected differently by the windscreen and eachprojection may be corrected independently in accordance with the presentdisclosure.

In more detail, FIG. 6 shows a first light source 601 a illuminating afirst array of spatially-modulating pixels 603 a. A first hologram isrepresented on the pixels 603 a. The first hologram comprises image dataand first lensing data 604 a. The image data is data representative of a2D image for projection. The first lensing data 604 a is data providinga first lensing effect. The spatially modulated light is incident upon afirst region 607 of a windscreen 602. The light is redirected by thewindscreen 602 to a first region 609 a of a viewing plane 609. Acorresponding optical path is provided for a second hologram. A secondlight source 601 b illuminates a second array of spatially-modulatingpixels 603 b. A second hologram is represented on the pixels 603 b. Thesecond hologram comprises the image data and second lensing data 604 b.The image data is the data representative of the 2D image forprojection. The second lensing data 604 b is data providing a secondlensing effect. In an embodiment, the first lensing data 604 a isdifferent to the second lensing data 604 b. The spatially modulatedlight is incident upon a second region 608 of the windscreen 602. Thelight is redirected by the windscreen 602 to a second region 609 b ofthe viewing plane 609. In an embodiment, the first region 609 a andsecond region 609 b of the viewing plane are substantially adjacentand/or do not overlap.

There is therefore provided a method of projection using an opticalelement having spatially variant optical power, the method comprising:combining Fourier domain data representative of a 2D image with Fourierdomain data having a first lensing effect to produce first holographicdata; combining the Fourier domain data representative of the 2D imagewith Fourier domain data having a second lensing effect to producesecond holographic data; spatially modulating light with the firstholographic data to form a first spatially modulated light beam andspatially modulating light with the second holographic data to form asecond spatially modulated light beam; redirecting the first and secondspatially modulated light beams using the optical element byilluminating a first region of the optical element with the firstspatially modulated beam and illuminating a second region of the opticalelement with the second spatially modulated beam; wherein the first andsecond lensing effects compensate for the optical power of the opticalelement in the first and second regions, respectively.

In an embodiment, the first lensing effect is different to the secondlensing effect and/or the first and second lensing effects areindependently selected or calculated. It can be understood that in thisrespect, different optical powers of the first and second regions of theoptical element may be individually compensated. It may be consideredthat the first and second holograms are independently-configured tocompensation for the spatially-varying and complex optical power of theoptical element.

Notably, this approach avoids the need for an expensive freeform mirrorby compensating for the complex optical power of the windscreen usingindividually-compensated holograms. Further advantageously, it can beunderstood that the system may be readily adjusted to compensate fordifferent viewing angles or different windscreen shapes, for example. Itcan further be appreciated that if the windscreen curvature is profiled,the system may dynamically respond to changes by selecting differentlensing data. In embodiments, there is therefore provided a head-updisplay which can be used in any vehicle without physical modification.

It can be understood that, in an embodiment, the first and secondlensing effects substantially negate the optical power of the opticalelement in the first and second regions, respectively.

In embodiments, the hologram is a phase-only hologram and the lensingeffect is provided by a phase-only lens. The phase-only hologram may becalculated in real-time or retrieved from a repository such as adatabase. The hologram may be calculated using a Gerchberg-Saxton typealgorithm or any other algorithm for generating a Fourier domainhologram. The skilled person will understand that the hologram mayequally be an amplitude hologram, or an amplitude and phase hologram,and the lensing effect may therefore be provided by amplitude hologram,or amplitude and phase hologram.

Optionally, because of the low numerical aperture of some SLMs,embodiments include an eye tracking mechanism to ensure the driver isable to see the holograms at all times (in the so called eye-box area).In these embodiments, a moving mirror or other light steering mechanism,coupled with the eye tracking system, is used. In an embodiment, theoptical element is arranged to redirect the first and second spatiallymodulated light beams to a viewing plane.

In a preferred system, each eye receives only one spatially modulatedbeam. The preferred separation of the beams at the viewing plane isdependent on the separation of eyes. In an embodiment, the first andsecond spatially modulated light beams are substantially adjacent at theviewing plane. If the beams overlap at the viewing plane, opticalinterference may occur. Therefore, in an embodiment, the first andsecond spatially modulated light beams do not overlap at the viewingplane.

The two holographic reconstructions are respectively compensated so aseach eye sees substantially the same image. If the two images differ,confusion may be caused. In an embodiment, the redirected firstspatially modulated light beam has a convergence or divergencesubstantially equal to that of the redirected second spatially modulatedlight beam.

The light may be spatially modulated using a spatial light modulatorsuch as a liquid crystal on silicon SLM. It can be understood that theholographic data is written to the SLM such that an incident plane waveof light is spatially modulated with the holographic data. In thisrespect, it may be considered that the pixels of the SLM “display” or“represent” the holographic data.

In an embodiment, spatial modulation is provided by representing thefirst and second holographic data on at least one spatial lightmodulator; and illuminating the at least one spatial light modulatorwith a plane wave to form the first and second spatially modulated lightbeams corresponding to the first and second holographic data,respectively.

Advantageous embodiments relate to a head-up display for a vehicle usingthe vehicle windscreen as an optical element to redirect light to theviewer. In this respect, the windscreen may be considered an opticalcombiner. That is, in embodiments, the optical element is a vehiclewindscreen. However, the skilled person will appreciate that the presentdisclosure is suitable for compensating for unwanted optical powerprovided by any optical component.

There is provided a corresponding projector having: processing meansarranged to combine Fourier domain data representative of a 2D imagewith Fourier domain data having a first lensing effect to produce firstholographic data; at least one spatial light modulator comprising anarray of pixels arranged to represent the first holographic data; anoptical element having spatially variant optical power, wherein theoptical element comprises a first region having a first optical power;wherein the first lensing effect compensates for the first opticalpower.

In a further advantageous embodiment: the processing means are furtherarranged to combine the Fourier domain data representative of the 2Dimage with Fourier domain data having a second lensing effect to producesecond holographic data; the at least one spatial light modulatorfurther comprises an array of pixels arranged to represent the secondholographic data; wherein the optical element further comprises a secondregion having a second optical power; and wherein the second lensingeffect compensates for the second optical power.

Embodiments utilise two SLMs to provide the two holographicreconstructions. However, if a sufficiently large SLM existed, the sameeffect would be possible using a single device with the individualholograms being written only to the area being viewed by the driver.That is, in other embodiments, different areas of the same SLM are usedto form the two holographic reconstructions. That is, in an embodiment,the at least one spatial light modulator comprises a first spatial lightmodulator comprising an array of pixels arranged to represent the firstholographic data and a second spatial light modulator comprising anarray of pixels arranged to represent the second holographic data.

The skilled person will understand that the light source may be part ofthe projector or an external component arranged to co-operate with theprojector. That is, in an embodiment, the projector further comprises alight source arranged to illuminate the at least one spatial lightmodulator with a plane wave.

In another embodiment a single light source is split using a beamsplitter or other optical splitter and is used to illuminate bothspatial light modulators.

It can be understood that a head-up display may display a variety ofinformation as known in the art. Holograms corresponding to all thepossible displays may be therefore be pre-calculated and stored in arepository, or calculated in real-time. In an embodiment, the projectorfurther comprises a repository of Fourier domain data representative ofa plurality of 2D images. Likewise, in embodiments, there is provided arepository of Fourier domain data having different lensing effects. Infurther embodiments, a look-up table of the optical power of the opticalelement as a function of position (e.g. x and y co-ordinates) isprovided so that the appropriate lensing data may be applied tocompensate for the optical element.

Embodiments described herein relate to Fourier holography by way ofexample only. The present disclosure is equally applicable to Fresnelholography in which Fresnel lens functions are applied duringcalculation of the hologram. FIG. 7 shows an example Fresnel holographicalgorithm for calculating the Fourier domain data representative of atarget image for projection.

The start condition 701 for the phase retrieval algorithm is that eachpixel has unity amplitude but a random phase provided by a random phaseseed function. A Fresnel phase function 703 is added to the phase data.The resultant amplitude and phase function is Fourier transformed 705.The target image (amplitude only) 709 is subtracted from the amplitudecomponent and a controllable gain 711 applied. The target image 709 isadded to the amplitude component and an inverse Fourier transform 715performed. The Fresnel lens function 717 is subtracted and the phasequantised 719. The resulting phase information forms the hologram 723. Afurther iteration of the loop may be performed by adding the Fresnellens function 721 again and repeating the Fourier transform 715 andsubsequent steps until an “acceptable” quality hologram is obtained.

The quality of the reconstructed hologram may be affect by the so-calledzero order problem which is a consequence of the diffractive nature ofthe reconstruction. Such zero-order light can be regarded as “noise” andincludes for example specularly reflected light, and other unwantedlight from the SLM.

This “noise” is generally focussed at the focal point of the Fourierlens, leading to a bright spot at the centre of a reconstructedhologram. Conventionally, the zero order light is simply blocked outhowever this would clearly mean replacing the bright spot with a darkspot.

However as the hologram contains three dimensional information, it ispossible to displace the reconstruction into a different plane inspace—see, for example, published PCT application WO 2007/131649incorporated herein by reference.

Alternatively and angularly selective filter could be used to removeonly the collimated rays of the zero order. Other methods of managingthe zero order may also be used.

Whilst embodiments described herein relate to displaying one hologramper frame, the present disclosure is by no means limited in this respectand more than one hologram may be displayed on the SLM at any one time.

For example, embodiments implement the technique of “tiling”, in whichthe surface area of the SLM is further divided up into a number oftiles, each of which is set in a phase distribution similar or identicalto that of the original tile. Each tile is therefore of a smallersurface area than if the whole allocated area of the SLM were used asone large phase pattern. The smaller the number of frequency componentin the tile, the further apart the reconstructed pixels are separatedwhen the image is produced. The image is created within the zerothdiffraction order, and it is preferred that the first and subsequentorders are displaced far enough so as not to overlap with the image andmay be blocked by way of a spatial filter.

As mentioned above, the image produced by this method (whether withtiling or without) comprises spots that form image pixels. The higherthe number of tiles used, the smaller these spots become. If one takesthe example of a Fourier transform of an infinite sine wave, a singlefrequency is produced. This is the optimum output. In practice, if justone tile is used, this corresponds to an input of a single cycle of asine wave, with a zero values extending in the positive and negativedirections from the end nodes of the sine wave to infinity. Instead of asingle frequency being produced from its Fourier transform, theprinciple frequency component is produced with a series of adjacentfrequency components on either side of it.

The use of tiling reduces the magnitude of these adjacent frequencycomponents and as a direct result of this, less interference(constructive or destructive) occurs between adjacent image pixels,thereby improving the image quality.

Preferably, each tile is a whole tile, although it is possible to usefractions of a tile.

Although embodiments relate to variants of the Gerchberg-Saxtonalgorithm, the skilled person will understand that other phase retrievalalgorithms may implement the improved method disclosed herein.

The skilled person will understand that the improved method disclosedherein is equally applicable to the calculation of a hologram used toform a three-dimensional reconstruction of an object.

Equally, the present disclosure is not limited to projection of amonochromatic image.

A colour 2D holographic reconstruction can be produced and there are twomain methods of achieving this. One of these methods is known as“frame-sequential colour” (FSC). In an FSC system, three lasers are used(red, green and blue) and each laser is fired in succession at the SLMto produce each frame of the video. The colours are cycled (red, green,blue, red, green, blue, etc.) at a fast enough rate such that a humanviewer sees a polychromatic image from a combination of the threelasers. Each hologram is therefore colour specific. For example, in avideo at 25 frames per second, the first frame would be produced byfiring the red laser for 1/75th of a second, then the green laser wouldbe fired for 1/75th of a second, and finally the blue laser would befired for 1/75th of a second. The next frame is then produced, startingwith the red laser, and so on.

An alternative method, that will be referred to as “spatially separatedcolours” (SSC) involves all three lasers being fired at the same time,but taking different optical paths, e.g. each using a different SLM, ordifferent area of a single SLM, and then combining to form the colourimage.

An advantage of the frame-sequential colour (FSC) method is that thewhole SLM is used for each colour. This means that the quality of thethree colour images produced will not be compromised because all pixelson the SLM are used for each of the colour images. However, adisadvantage of the FSC method is that the overall image produced willnot be as bright as a corresponding image produced by the SSC method bya factor of about 3, because each laser is only used for a third of thetime. This drawback could potentially be addressed by overdriving thelasers, or by using more powerful lasers, but this would require morepower to be used, would involve higher costs and would make the systemless compact.

An advantage of the SSC (spatially separated colours) method is that theimage is brighter due to all three lasers being fired at the same time.However, if due to space limitations it is required to use only one SLM,the surface area of the SLM can be divided into three equal parts,acting in effect as three separate SLMs. The drawback of this is thatthe quality of each single-colour image is decreased, due to thedecrease of SLM surface area available for each monochromatic image. Thequality of the polychromatic image is therefore decreased accordingly.The decrease of SLM surface area available means that fewer pixels onthe SLM can be used, thus reducing the quality of the image. The qualityof the image is reduced because its resolution is reduced.

In embodiments, the SLM is a Liquid Crystal over silicon (LCOS) device.LCOS SLMs have the advantage that the signal lines, gate lines andtransistors are below the mirrored surface, which results in high fillfactors (typically greater than 90%) and high resolutions.

LCOS devices are now available with pixels between 4.5 μm and 12 μm.

The structure of an LCOS device is shown in FIG. 8.

A LCOS device is formed using a single crystal silicon substrate (802).It has a 2D array of square planar aluminium electrodes (801), spacedapart by a gap (801 a), arranged on the upper surface of the substrate.Each of the electrodes (801) can be addressed via circuitry (802 a)buried in the substrate (802). Each of the electrodes forms a respectiveplanar mirror. An alignment layer (803) is disposed on the array ofelectrodes, and a liquid crystal layer (804) is disposed on thealignment layer (803). A second alignment layer (805) is disposed on theliquid crystal layer (404) and a planar transparent layer (806), e.g. ofglass, is disposed on the second alignment layer (805). A singletransparent electrode (807) e.g. of ITO is disposed between thetransparent layer (806) and the second alignment layer (805).

Each of the square electrodes (801) defines, together with the overlyingregion of the transparent electrode (807) and the intervening liquidcrystal material, a controllable phase-modulating element (808), oftenreferred to as a pixel. The effective pixel area, or fill factor, is thepercentage of the total pixel which is optically active, taking intoaccount the space between pixels (801 a). By control of the voltageapplied to each electrode (801) with respect to the transparentelectrode (807), the properties of the liquid crystal material of therespective phase modulating element may be varied, thereby to provide avariable delay to light incident thereon. The effect is to providephase-only modulation to the wavefront, i.e. no amplitude effect occurs.

A major advantage of using a reflective LCOS spatial light modulator isthat the liquid crystal layer can be half the thickness than would benecessary if a transmissive device were used. This greatly improves theswitching speed of the liquid crystal (a key point for projection ofmoving video images). A LCOS device is also uniquely capable ofdisplaying large arrays of phase only elements in a small aperture.Small elements (typically approximately 10 microns or smaller) result ina practical diffraction angle (a few degrees) so that the optical systemdoes not require a very long optical path.

It is easier to adequately illuminate the small aperture (a few squarecentimeters) of a LCOS SLM than it would be for the aperture of a largerliquid crystal device. LCOS SLMs also have a large aperture ratio, therebeing very little dead space between the pixels (as the circuitry todrive them is buried under the mirrors). This is an important issue tolowering the optical noise in the replay field.

The above device typically operates within a temperature range of 10° C.to around 50° C., with the optimum device operating temperature beingaround 40° C. to 50° C., depending however on the LC composition used.

Using a silicon backplane has the advantage that the pixels areoptically flat, which is important for a phase modulating device.

Whilst embodiments relate to a reflective LCOS SLM, the skilled personwill understand that any SLM can be used including transmissive SLMs.

The invention is not restricted to the described embodiments but extendsto the full scope of the appended claims.

The invention claimed is:
 1. A method of projection comprising: combining Fourier domain data representative of a 2D image with Fourier domain data having a first lensing effect to produce first holographic data; and combining the Fourier domain data representative of the 2D image with Fourier domain data having a second lensing effect to produce second holographic data; and, concurrently, providing a first spatially modulated light beam to a first region of a viewing plane by a method comprising spatially modulating light with the first holographic data to form the first spatially modulated light beam; redirecting the first spatially modulated light beam using an optical combiner by illuminating a first region of the optical combiner with the first spatially modulated beam, the optical combiner having a spatially variant optical power, the optical combiner redirecting the first spatially modulated light beam to the first region of a viewing plane, wherein the first lensing effect compensates at the first region of the viewing plane for the first optical power of the optical combiner in the first region; and providing a second spatially modulated light beam to a second region of a viewing plane, the second region of the viewing plane being different from the first region of the viewing plane, by a method comprising spatially modulating light with the second holographic data to form the second spatially modulated light beam; redirecting the second spatially modulated light beam using the optical combiner by illuminating a second region of the optical combiner with the second spatially modulated beam, the second region of the optical combiner being different from the first region of the optical combiner and having an optical power, the optical combiner redirecting the second spatially modulated light beam to the second region of the viewing plane; wherein the second lensing effect compensates at the second region of the viewing plane for the optical power of the optical combiner in the second region.
 2. The method of claim 1 wherein the first lensing effect negates at the first region of the viewing plane the optical power of the first region of the optical combiner and the second lensing effect negates at the second region of the viewing plane the optical power of the second region of the optical combiner.
 3. The method of claim 1 wherein the first and second spatially modulated light beams are adjacent at the viewing plane.
 4. The method of claim 1 wherein the first and second spatially modulated light beams do not overlap at the viewing plane.
 5. The method of claim 1 wherein the redirected first spatially modulated light beam has a convergence or divergence substantially equal to that of the redirected second spatially modulated light beam.
 6. The method of claim 1 wherein spatially modulating light with the first holographic data to form a first spatially modulated light beam and spatially modulating light with the second holographic data to form a second spatially modulated light beam comprises: representing the first and second holographic data on at least one spatial light modulator; illuminating the at least one spatial light modulator with a plane wave to form the first and second spatially modulated light beams corresponding to the first and second holographic data, respectively.
 7. The method of claim 1 wherein the optical combiner is a vehicle windscreen.
 8. A method as claimed in claim 1, wherein the optical power of the first region of the optical combiner results from a curvature of the optical combiner in the first region thereof.
 9. A method as claimed in claim 8, wherein the optical power of the second region of the optical combiner results from a curvature of the optical combiner in the second region thereof.
 10. A method as claimed in claim 1, wherein the optical power of the first region of the optical combiner is different from the optical power of the second region of the optical combiner.
 11. A method as claimed in claim 1, wherein the redirected first spatially modulated light beam is received by a first eye of a viewer but not by a second eye of a viewer, and the redirected second spatially modulated light beam is received by the second eye of the viewer but not by the first eye of the viewer.
 12. A method as claimed in claim 1, wherein the redirected first spatially modulated light beam does not overlap with the redirected second spatially modulated light beam at the viewing plane.
 13. A method as claimed in claim 1, wherein the redirecting of the first spatially modulated light beam is a reflecting of the first spatially modulated light beam, and the redirecting of the second spatially modulated light beam is a reflecting of the second spatially modulated light beam.
 14. A projector comprising: a computer processor arranged to combine Fourier domain data representative of a 2D image with Fourier domain data having a first lensing effect to produce first holographic data, and to combine the Fourier domain data representative of the 2D image with Fourier domain data having a second lensing effect to produce second holographic data; one or more spatial light modulators, the one or more spatial light modulators together comprising a first array of pixels arranged to represent the first holographic data, the first array of pixels of the spatial light modulator being configured to spatially modulate light with the first holographic data to provide a first spatially modulated light beam, and a second array of pixels arranged to, concurrently with the representation of the first holographic data, represent the second holographic data, the second array of pixels of the spatial light modulator being configured to spatially modulate light to provide a second spatially modulated light beam; an optical combiner having spatially variant optical power, wherein the optical combiner comprises a first region having an optical power, the optical combiner being configured to be illuminated by the first spatially modulated light beam in the first region thereof and redirect the first spatially modulated light beam to a first region of a viewing plane, and a second region having a second optical power, the second region of the optical combiner being different than the first region of the optical combiner, the optical combiner being configured to be, concurrent with the illumination of the first region thereof, illuminated by the second spatially modulated light beam in the second region and to redirect the second spatially modulated light beam to a second region of the viewing plane different from the first region of the viewing plane; wherein the first lensing effect compensates at the first region of the viewing plane for the first optical power of the first region of the optical combiner, and the second lensing effect compensates at the second region of the viewing plane for the optical power of the second region of the optical combiner.
 15. A projector as claimed in claim 14 wherein the one or more spatial light modulators comprise a first spatial light modulator comprising the first array of pixels and a second spatial light modulator comprising the second array of pixels.
 16. A projector as claimed in claim 14 further comprising a light source arranged to illuminate the at least one spatial light modulator with a plane wave.
 17. A projector as claimed in claim 14 further comprising a repository of Fourier domain data representative of a plurality of 2D images, the repository being configured to provide the Fourier domain data to the computer processor.
 18. A projector as claimed in claim 14 wherein the optical combiner is a vehicle windscreen.
 19. A projector as claimed in claim 14, wherein the optical power of the first region of the optical combiner results from a curvature of the optical combiner in the first region thereof.
 20. A projector as claimed in claim 19, wherein the optical power of the second region of the optical combiner results from a curvature of the optical combiner in the second region thereof.
 21. A projector as claimed in claim 14, wherein the optical power of the first region of the optical combiner is different from the optical power of the second region of the optical combiner.
 22. A projector as claimed in claim 14, wherein the first region of the optical combiner is configured to redirect the first spatially modulated light beam by reflecting it, and the second region of the optical combiner is configured to redirect the second spatially modulated light beam by reflecting it. 